Generally, in a fluorescent lamp, a pair of hot-cathode-type electrodes is provided at both ends of the lamp, a phosphor is formed in a layered manner on a surface of a glass tube, and a protective film of, e.g., aluminum oxide is formed between the glass tube and the phosphor layer.
As to the inside of the glass tube, its evacuation is carried out with the entire glass tube being heated during the exhaustion process. In order to enhance the exhaustion efficiency during this exhaustion process, a treatment called “argon flash” is carried out.
The “argon flash” is a method in which argon gas is sealed into the fluorescent lamp during the exhaustion process, residual impure gas occluded by the phosphor and the protective film is heated and discharged into the lamp, and the evacuation is carried out again after diluting the above gas with argon gas. The “argon flash” may be repeated several times.
The “argon flash” allows the residual impure gas to be effectively reduced and the ultimate degree of vacuum inside the glass tube to be increased, both in a limited exhaustion process.
However, since a fluorescent lamp is an industrially manufactured product, it is difficult to bring it to a complete vacuum and the ultimate degree of vacuum (admissible residual gas level) is set within a range where no disturbances occur during the actual use of the lamp. Namely, while it is desirable that the manufacturing process of the fluorescent lamp is carried out in high vacuum, a thus manufactured lamp is very expensive. Thus, the lamp is manufactured with a degree of vacuum where a defect does not occur during the actual use.
If a large quantity of impure gas exists in the discharge space of the glass tube of a fluorescent lamp, the lamp voltage (discharge maintaining voltage) rises and becomes higher than the voltage fed from the lighting circuit, the lamp cannot discharge and extinguishes. This phenomenon is called “turning off”.
It is known that the residual impure gas causes the lamp voltage to rise. For example, there has been suggested a technology which applies the turning off phenomenon in that it “incorporates a thin tube made of glass with impure gas sealed inside it” into a part of the arc tube so that, at the end of the lifespan of the fluorescent lamp, the discharging is stopped (see e.g. Patent Document 1).
Another publication describes the phenomenon that the above impure gas has an adverse influence (see e.g. Patent Document 2).
Generally, fluorescent lamps are designed by taking the temperature rise in a lighting instrument into consideration. Thus, fluorescent lamps and the lighting instruments are designed so that, even if the lamp ambient temperature rises, no defect occurs.
Generally, a temperature from 0° C. to 60° C. is described as the expected ambient temperature of the fluorescent lamp, (see e.g. Non-Patent Document 1).
As shown in FIGS. 2 and 12 of Non-Patent Document 1, it has been the technical common knowledge to a person skilled in the art that, at temperatures exceeding room temperature (25° C.), the lamp voltage of a fluorescent lamp drops when the ambient temperature rises.
The inventors have strived for the miniaturization of lighting instruments and examined the combination of a miniaturized compact fluorescent lamp and a miniaturized lighting instrument. Then, the “turning off” phenomenon occurred due to the temperature rise in the lamp and the temperature rise in the lighting instrument. This seemed to be caused by the residual impure gas according to the prior art, and the degree of vacuum was improved during the manufacturing process. However, this level was far higher than the one in the prior art. Even by lowering the residual impure gas concentration, the problem of the turning off was not solved.
When the details of the problem were further examined, it turned out that, though the impure gas was removed, the lamp voltage rose and the turning off occurred when the temperature of the lamp became high. It was found that the cause of this lies in that, in a region with a higher temperature (exceeding 60° C.) than what was described in the above Non-Patent Document 1, there exists a region where the mercury vapor pressure increases and, in conjunction with this, the lamp voltage also increases steeply. Thus, the inventors realized that the phenomenon of the turning off cannot be improved solely by simply removing the impure gas.
Such a phenomenon seemed to become a serious problem, because the environments of use with a rising lamp temperature were expected to increase from then on because of the miniaturization of lighting instruments, use of multiple lamps in downlights (illuminating directly downward with small lights or small light sources embedded in the ceiling, and also used as auxiliary light), or changes in the environments in which the lighting instruments were installed. This phenomenon includes methods of abnormal use, which cause the temperature in a lighting instrument to rise beyond expectation, wherein such an instrument is covered during construction work by a heat insulating material in a space above the ceiling, or the lower surface is shielded by a certain member.
These phenomena are not likely to occur in straight-tube fluorescent lamps which have been mainly used so far. This is because the straight-tube fluorescent lamp has the features that the bulb wall loading is low and the lamp temperature does not easily rise and also because heat is not easily contained in the lighting instrument due to its shape.
A product group of 3U-form single-base fluorescent lamps called FHT, which have been commercialized and, as seen from their past, have a large electric power consumption such as 24 W, 32 W, and 57 W, is expected to increase in the future and has a large bulb wall loading.
Three or four lamps are lit simultaneously in one lighting instrument, and because they are used in downlamps, heat is easily contained in the reflector plate. In some cases, the lighting instrument itself is covered with a heat insulating material during the construction, and the environmental temperature of fluorescent lamps is expected to become higher and higher. As an example of an FHT multi-lamp downlight instrument, four FHT42 lamps are lit simultaneously in the same reflector plate.
The inventors removed the impure gas and experimentally produced many compact fluorescent lamps with impure gas quantities within the range where the rising of the lamp voltage due to the impure gas can be sufficiently restricted even during an operation in a high temperature region. Further, they examined a means for restricting the rising of the lamp voltage accompanying the mercury vapor pressure increase.
As a result, the inventors conceived of utilizing an unsaturated mercury vapor discharging by lighting a mercury vapor in an unsaturated region.
Utilizing an unsaturated mercury vapor discharging has been suggested in the past. As described in Non-Patent Document 1, generally, fluorescent lamps have the problem that the saturated vapor pressure of mercury changes according to the ambient temperature and the change of the mercury evaporation amount causes the discharge characteristic to change, so that the brightness, too, is changed. Here, utilizing the unsaturated mercury vapor discharging is a method suggested for the purpose of obtaining a fluorescent lamp whose brightness does not change according to the ambient temperature.
Concretely, the present invention is intended for solving the problem that the brightness of a fluorescent lamp such as a reading light source of a facsimile changes depending on whether a room temperature is high or low, so that the light receiving quantity of a reading CCD (Charge Coupled Device) changes. It has been suggested to set the amount of mercury sealed into the lamp in such a manner that the mercury in the tube is unsaturated at temperatures below the lower limit of the room temperature (see e.g. Patent Document 3).
In this manner, the mercury in the tube of the fluorescent lamp is unsaturated and completely gasified in the normal-use temperature range. Therefore, no further mercury vapor pressure change occurs and the temperature characteristics of the fluorescent lamp become constant. Thus, there is the advantage that the lamp characteristics do not change in the temperature region normally used. Further, the quantity of the sealed mercury is small.